The passage by Congress of laws to reduce emissions of certain gases from automobiles led to the introduction by the automobile manufacturers of various control systems. These control systems minimize exhaust emissions and fuel consumption, provide optimal driveability for all operating conditions, minimize evaporative emissions, and provide system diagnosis when malfunctions occur.
Control systems developed by automobile manufacturers include the Positive Crankcase Ventilation (PCV) system, Exhaust Gas Recirculation (EGR) system, and fuel evaporation emission control system. These are effective in blocking the escape of emissions from such areas as the carburetor, fuel tank, and crankcase of automotive vehicles.
Another system was needed to reduce emissions of hydrocarbon (HC) and carbon monoxide (CO). These compounds are generated as fuel is burned in an engine's cylinders and are released into the atmosphere with the exhaust. The catalytic converter was developed to neutralize these compounds. Catalytic converters have been installed on practically every vehicle having a gasoline engine that has been manufactured since 1975.
To further meet emissions standards imposed by Congress, automobile manufacturers have developed an exhaust gas oxygen sensor which is positioned in the path of the exhaust gases. The exhaust gas oxygen sensor provides a feedback signal of the oxygen content of the exhaust gases from which it can be determined if the air/fuel mixture being supplied to the engine is rich or lean. This feedback signal is transmitted to a fuel control system in the automobile wherein a fuel control signal is switched around stoichiometric air/fuel ratio. The fuel control signal is used to control the fuel mixture delivered to the engine, thereby controlling the amount of emission constituents exiting from the automobile's tailpipe to a minimum. The stoichiometric air/fuel mixture is approximately 14.7 parts air and one part gasoline.
The exhaust gas oxygen sensor needs to operate at a temperature over 670.degree. F. to provide an accurate signal. In an effort to reduce the time required to achieve operating temperature, a resistive element heater has been added to the sensor. However, as the sensor ages, the heater's effectiveness degrades. In addition, the heater is always susceptible to electrical system problems, such as, open circuits, poor connections, harness connector corrosion, etc.
When the Heated Exhaust Gas Oxygen (HEGO) sensor degrades or fails, a lean or rich bias signal, depending on the type of HEGO sensor utilized, is imposed onto the feedback signal generated by the HEGO. In the case where the HEGO sensor utilized imposes a rich bias onto the feedback signal when the sensor degrades or fails, the fuel control system reduces the amount of fuel being delivered to the engine resulting in a leaner air/fuel mixture which causes more production of NO.sub.x during combustion with minimum impact on the production of CO and HC. Thus, the engine may not meet the government-imposed emissions standards. Conversely, if the HEGO sensor utilized imposes a lean bias when the sensor fails, the fuel control system increases the amount of fuel delivered to the engine resulting in a richer air/fuel mixture that causes more production of CO in the exhaust gas.